This invention relates to an orifice metering apparatus and a method of fabricating same and, more particularly, to such an apparatus and method in which an orifice is centered in a substantially coaxial relation in a pipeline to obtain precise fluid flow measurements.
Through the oil and gas industry, these is a need for accurate, economical measurement of process fluids, such as natural gas, or the like, flowing through a pipeline. The most popular apparatus for realizing this is an orifice meter which is relatively versatile and low in cost compared to other flow meters.
In general, an orifice meter consists of a section of straight pipe which is inserted in the pipeline and which contains a constrictive device, usually in the form of an orifice plate, which reduces the cross-sectional area of the flow stream. In order to maintain a steady flow of fluid throughout the pipeline, the velocity of the fluid must increase through the orifice. This increase in kinetic energy must be accompanied by a decrease in another form of energy which is indicated by a decrease in static pressure or differential pressure. The differential pressure between the upstream and downstream section of the orifice plate, due to the energy exchange, can be used as a measure of the difference in fluid velocity between these sections and is the principle behind flow meters of this type. To this end, the differential pressure is sensed, and when combined with correction factors for the meter itself and the physical characteristics of the fluid being measured, allows computation of the rate of flow, usually by a chart recorder and/or flow computers.
Unfortunately, due to the relatively wide range of tolerances in the several parts making up a orifice meter, it is difficult to obtain flow measurements with a great deal of accuracy. Although this can be overcome by calibrating the meters, the calibration process is time-consuming and costly.
The above problems with orifice meters are compounded by recent regulations requiring, in certain circumstances, that orifice meters be constructed to much tighter dimensional and configurational requirements than in the past.